Recordings of neuronal action potentials (NAPs) evoked by close-by applied electrical stimulation provide a signal mixture containing two major components: (1) the desired NAPs, and (2) the applied electrical stimulus. The removal of the unintentional recorded stimulus waveform (also called the stimulus artifact) from this mixture has proven to be a difficult task.
Various techniques have been used to eliminate or reduce the stimulus artifact from recordings of evoked compound action potentials (ECAP) in neuroprosthetic devices such as cochlear implants. In the alternating stimulation approach, two recordings are performed using anodic-cathodic and cathodic-anodic biphasic stimulation pulses. See Eisen M D, Franck K H, Electrically Evoked Compound Action Potential Amplitude Growth Functions and HiResolution Programming Levels in Pediatric CII Implant Subjects, Ear & Hearing 2004, 25(6):528-538, incorporated herein by reference. The NAPs are assumed to be independent of the polarity of the first phase, so the stimulus artifact can be averaged out. But alternating stimulation creates an undesired increase in measurement time. Moreover, the assumption of phase-invariance of an NAP does not hold completely.
In the masker probe method, a second probe pulse is sent within the neuron's refractory time which allows a template for the stimulus artifact to be measured. See Brown C, Abbas P, Gantz B, Electrically Evoked Whole-Nerve Action Potentials: Data From Human Cochlear Implant Users, Journal of the Acoustical Society of America 1990, 88(3):1385-1391, and Miller C A, Abbas P J, Brown C J, An Improved Method Of Reducing Stimulus Artifact In The Electrically Evoked Whole-Nerve Potential, Ear & Hearing 2000, 21(4):280-290; which are incorporated herein by reference. As with alternating stimulation, the masker probe approach requires an undesirable increase in measurement time. In addition, some of the nerves typically are not in a refractory state.
A triphasic pulse can applied and the amplitude of the third phase selected so that the total electrical charge introduced equals zero. See Zimmerling M, Messung des elektrisch evozierten Summenaktionspotentials des Hörnervs bei Patienten mit einem Cochlea-Implantat, PhD thesis Universität Innsbruck, Institut für Angewandte Physik, 1999, and Schoesser H, Zierhofer C, Hochmair E S, Measuring Electrically Evoked Compound Action Potentials Using Triphasic Pulses For The Reduction Of The Residual Stimulation Artefact, In Conference On Implantable Auditory Prostheses, 2001; which are incorporated herein by reference. But the use of a triphasic pulse leads to an increased delay between the stimulus portion that triggers the NAPs and the beginning of the recording.
Another method records the response to a stimulus at sub-threshold level to measure a template for the stimulus artifact which is then scaled to supra-threshold levels and subtracted from the recorded signal mixture. See Miller C A, Abbas P J, Rubinstein J T, Robinson B, Matsuoka A, Woodworth G, Electrically Evoked Compound Action Potentials Of Guinea Pig And Cat: Responses To Monopolar, Monophasic Stimulation, Hearing Research 1998, 119(1-2):142-154; incorporated herein by reference. One disadvantage with that approach is that the artifact does not scale linearly with increasing amplitudes.